The present invention relates to a control for operation of an electrically powered lift truck.
Such lift trucks normally use hydraulically actuated rams to lift the fork carriage, to tilt the mast, and to operate a variety of attachments such as sideshifters, clamps, rotators and the like. The hydraulic system for the rams normally uses a single hydraulic pump of the fixed-displacement type driven by an electric motor using the truck battery as its power source.
The various hydraulic functions require widely differing rates of hydraulic fluid flow. The tilt and attachments require relatively low rates of flow, while the lift function requires a much higher flow rate. The tilt and attachment flow requirements have remained relatively constant over the years, whereas the lift speed has increased considerably over recent years, thus increasing the range of flow rate required for operation.
The motor and pump must, of course, be designed to supply the maximum flow required for the lift function. If the motor is connected directly across the battery, as is often the case, excess fluid flow is produced when the tilt and attachment rams are operated. The excess fluid flow has to be dumped back to the supply tank at a high relief pressure which represents a considerable waste of energy and which causes an undesirable heating of the fluid. A similar situation occurs when the lift is operated at less than full speed except that in this case the excess oil is passed back to the supply tank at system pressure.
Electronic controls have been developed for electric motors wherein the motor is connected to the battery through an electronic switch means, typically a thyristor in the form of a silicon-controlled rectifier. The switch means is repeatedly closed and opened, with the ratio of closed to open time being controllable to regulate the average voltage delivered to the motor from the battery so that the speed thereof can be controlled.